Fluorine-Free Sulfonated Poly(sulfone triazole)s with a Pendant Phosphaphenanthrene Moiety for Proton Exchange Membrane Applications

Fluorinated proton exchange membranes (PEMs), such as Nafion, are the current state-of-the-art polymers, but they pose environmental challenges, driving the need for more sustainable fluorine-free alternatives. In this work, a phosphaphenanthrene-based bisalkyne monomer, 6-(1,1-bis(4-(prop-2-yn-1-yloxy)phenyl)ethyl)dibenzo[c,e][1,2]oxaphosphinine 6-oxide (DPAK), has been prepared. Utilizing this monomer (DPAK), a set of sulfonated poly(sulfone triazole)s with a pendant phosphaphenanthrene unit in the hard (hydrophobic) and soft (hydrophilic) segments of the copolymer have been designed and synthesized by the “click” reaction with various degrees of sulfonation values. The chemical structures of the copolymers were confirmed by various spectroscopic (FTIR, NMR, and XPS) techniques. The salt-form sulfonated poly(sulfone triazole) (SODPSNa-XX) copolymers exhibited high solubility in polar aprotic solvents. The acid-form sulfonated poly(sulfone triazole) (SODPSH-XX) copolymers exhibited high thermal, mechanical, and viscoelastic properties. Despite the absence of fluorinated moieties, the SODPSH-XX membranes possess moderate water absorption properties and show high-dimensional stability. The morphological (AFM, FESEM, and HRTEM) investigations of the SODPSH-XX membranes indicate the formation of an interconnected and well-segregated phase morphology, which created interconnected ionic cluster-like channels for the agile proton migration process. The SODPSH-90 membrane demonstrates much higher proton conductivity at 80 and 90 °C (σ: 100 and 112 mS/cm) than the few earlier reported fluorine-free sulfonated PEMs with comparable IEC_w values. The SODPSH-XX copolymers exhibited high durability, mainly associated with bulky cyclic phosphaphenanthrene and sulfonyl units in the copolymer backbone. Also, the SODPSH-70 to -90 membranes demonstrated better oxidative stability in Fenton’s reagent at 80 °C.